Maspin is a 42 kDa protein that belongs to the serpin family of protease inhibitors. As a cytoplasmic protein, maspin associates with secretory vesicles and the cell surface (Pemberton et al., J. Histochem. Cytochem 45:1697-1706, 1997) and is expressed in normal human mammary epithelia cells (Zou et al., Science 263:526-529, 1994).